Oscillators are used to produce an output signal at a particular frequency. The frequency of the output signal is dictated by the resonant frequency of a resonant circuit which is present in the oscillator. In this context, it is possible to use suitable measures to influence the resonant frequency of the resonant circuit by adding or disconnecting frequency-changing elements.
In general, a distinction is drawn in this context between a voltage controlled oscillator (VCO) and a digitally controlled oscillator (DCO). In voltage controlled oscillators, the frequency of the output signal is influenced by means of a continuously alterable DC voltage. The effect achieved by this is a uniform and continuous frequency change in the output signal. In a digitally controlled oscillator, a digital control word is taken as a basis for adding the frequency-changing elements to the resonant circuit or for removing them from it. This changes the output frequency of a digitally controlled oscillator in discrete-value steps.
FIG. 17.36, page 643, of the document by Thomas H. Lee: The Design of CMOS Radio-frequency integrated circuits, 2nd Edition 2004, shows a simple illustration of an oscillator at a fixed output frequency. This frequency is determined by the resonant circuit comprising the two coils L and the capacitance C.
Additional switchable frequency-changing elements allow the output frequency of the oscillator shown to be influenced. FIG. 5 shows a known resonant circuit having a frequency-changing element. In this case, the frequency-changing element comprises the two series-connected capacitors C1 and C2. These are arranged in parallel with the coil L1. The output nodes O1 and O2 are used to tap off a differential signal at the resonant frequency of the resonant circuit shown. Arranged between the two series-connected capacitors C1 and C2 of the frequency-changing element are the transistors T1 and T2. A first connection of each transistor is connected to a respective connection of one of the capacitors C1, C2. In addition, the first connections of the two transistors are connected to the ground potential connection M1 via a respective resistor R1. Similarly, the substrate connections of the two transistors T1 and T2 are coupled to the ground potential M1. The respective second connections of the transistors are coupled to one another.
The transistors T1 and T2 are used to switch the frequency-changing element into the resonant circuit during operation of the resonant circuit using a control signal on the control connections SC1, and in this way to alter the resonant frequency of the resonant circuit.
If the two transistors T1 and T2 have been switched to an on state, the transistors behave like resistors having low resistance values. The two capacitors C1, C2 have then been switched into the resonant circuit. If the capacitance values of the two capacitors C1 and C2 are the same, the total capacitance of the frequency-changing element is C/2. If the two transistors T1 and T2 have been switched to a nonconductive state, they behave like an open switch having a high nonreactive resistance. In such a case, the total capacitance is obtained through the following expression:
      C    tot    =            1      2        ⁢                  C        *                  C          par                            C        +                  C          par                    
In this case, the capacitance value Cpar is a parasitic capacitance induced by the two transistors. This is a characteristic of the transistors used and is dependent on the design, the nature and the material of the transistors, for example. Since the transistors should have the lowest possible resistance in the closed state, known as the “on state”, their dimensions are relatively large. It follows from this that the MOS field-effect transistors used usually also have large parasitic capacitances. This also reduces an effective capacitance change between the turned-on state, the “on state”, and the turned-off state, the “off state”.
There is therefore a need to make improvements to resonant circuits and oscillators.